I think the latest evidence is that actually the most benefit is derived from taking it pre conception BUT it has been proven to be beneficial througout the first trimester as well. Many, many women only take it after the pregnancy test (I started at about 6 weeks with dd1) and for others they only manage about a month before falling pregnant if they conceive quickly. So DON'T WORRY about not taking it before. If you are in good health and eating a balanced diet your body should have good folate levels in any case.
Folic acid added to breakfast cereal and bread for this reason I believe. Didn't know I was pregnant with DS2 until 11 weeks but midwife reassured me about the 'fortified with vitamins' thing and also normal diet with broccoli etc all folate rich!